1. Field of the Invention
The present invention relates to a light quantity adjustment device that is provided in an image pickup apparatus such as a video camera, a monitoring camera, and a digital camera and that is suitable to control the quantity of light to be incident on an image pickup element using an aperture mechanism and a neutral density (ND) filter.
2. Description of the Related Art
Image pickup apparatuses such as video cameras and digital cameras use a light quantity adjustment device that uses a plurality of aperture blades to form an aperture opening. In operation, the light quantity adjustment device varies the shape or area of the aperture opening to adjust the quantity of light incident on an image pickup element. Adjusting the light quantity by varying the area of the aperture opening reduces the cross-sectional area (diameter) of the aperture opening when a luminous object is to be photographed. When the cross-sectional area of aperture opening is reduced excessively, diffraction of light tends to degrade optical performance of the image pickup apparatus. Thus, an ND filter is used to reduce the quantity of light incident on the image pickup element without reducing the cross-sectional area of the aperture opening to the point that it becomes too small. Examples of such conventional light quantity adjustment devices have been previously disclosed.
Japanese Patent application Laid-Open No. 8-43878 discloses an aperture device in which an ND filter with a transmittance that varies radially stepwise or continuously is provided to a part of each of two aperture blades. The transmittance of the ND filter is set to become gradually lower from the optical axis toward the outer side in a radial direction. In this manner, the aperture opening is shielded by the ND filter to reduce the transmissivity from the center of the optical axis toward the outside in the radial direction, so that the aperture opening is not likely to become too small.
U.S. Patent Application Publication No. 2008/0037092 discloses a light quantity adjustment device in which an aperture and an ND filter are driven separately and in which the ND filter includes a transparent portion sized to fully cover the entirety of an aperture opening when the opening has a predetermined area. When the aperture opening has the predetermined area, the ND filter is controlled so as to move without a halt from a position retracted from the aperture opening to a position at which the transparent portion fully covers the entirety of the aperture opening with the predetermined area.
In the aperture device according to Japanese Patent Laid-Open No. 8-43878, the degree of overlap of the ND filter with the aperture opening is adjusted to reduce the difference in light quantity that occurs on the plane of focus, and hence unevenness in light quantity in the photographing screen is prevented. In the case where the entirety of the aperture opening is not covered by the ND filter, a phase difference may be caused in the direction perpendicular to the distal-end surface of the ND filter depending on the presence or absence of the ND filter in the aperture opening, which tends to reduce resolving power in the direction perpendicular to the distal-end surface of the ND filter. In the direction parallel to the distal-end surface of the ND filter, on the other hand, no phase difference is caused, and therefore resolving power is not reduced. This results in a difference in resolving power between the direction parallel to the distal-end surface of the ND filter and the direction perpendicular thereto. In addition, the end surface of the ND filter is formed to have a straight shape, which causes diffraction of light in the direction perpendicular to the straight shape of the end surface of the ND filter when the aperture opening is not covered by the ND filter. Therefore, a straight light streak with a high intensity may be caused when a luminous object enters the photographing screen. When such a light streak extending in the vertical direction of the photographing screen discussed above is caused, the light streak may look as if it was a smear extending in the vertical direction of the photographing screen, even though no smear would be caused when a CMOS sensor, rather than a CCD sensor, is used in an image pickup apparatus.
In the light quantity adjustment device according to U.S. Patent Application Publication No. 2008/0037092, a state with the end surface of the ND filter located in the aperture opening is determined as an unused region. Therefore, unlike the device according to Japanese Patent Laid-Open No. 8-43878, no difference in resolving power is caused between the direction of the distal-end surface of the ND filter and the direction perpendicular thereto. In addition, the distal-end surface of the ND filter is formed to have a convex shape, which does not cause a light streak with a high intensity extending in the vertical direction of the photographing screen similar to a smear. In the device according to U.S. Patent Application Publication No. 2008/0037092, the size of the ND filter in the driving direction tends to be large by an amount allowing passage of the unused region. In order to reduce the size of the ND filter with the unused region maintained, it is necessary to reduce the region (aperture opening) for reducing the quantity of light that passes through the aperture opening.
This may cause degradation in image quality due to diffraction with a small aperture. Further, if control is performed such that the end surface of the ND filter momentarily passes through the aperture opening in order to avoid the unused region, an unnatural image may be momentarily captured while recording a movie, for example.
The present invention provides a light quantity adjustment device capable of reducing occurrence of a light streak extending in the vertical direction of a photographing screen, reducing the difference in resolving power between a particular direction with respect to the photographing screen and the direction perpendicular thereto, and restricting the quantity of light that passes through an aperture opening, and also provides an image pickup apparatus having the light quantity adjustment device.